Seamless aluminum tubes, which include aluminum or aluminum alloy tubes have been widely used in the industry, and are considered an important element in the optical, aerospace, and mechanical industries. Conventionally, seamless aluminum tubes are made involving a piercing process through solid billets to form the pierced hollow central portions. The conventionally piercer-type extrusion process for making seamless aluminum tubes often suffers from heavy consumption of piercer rods due to breakage and/or constant rubbing and abrading between the piercer rod and the billet, thus resulting increased equipment cost relative to the conventional direct extrusion molding process. To minimize the consumption of piercer rods, improved processes have been developed which utilize hollow billets to making seamless tubes. More recently, new techniques have also been developed using a centrifugal or a semi-continuous casting process to make hollow billets.
However, it was discovered by the inventors that, although the use of hollow billets presents several distinct advantages over the conventional process, it also introduces new problems. More specifically, if the hollow billets are not immediately processed after they are formed, an oxide layer will form on the inner surface of the hollow billet. Such an oxide layer, due to its stiffness, often introduces defects in the seamless tubes and causes damages to the piercer rod. Furthermore, the present seamless tubing extrusion apparatus designed for extruding hollow billets does not allow easy switch to the mode of extruding solid billets. The latter disadvantage limits the application of an extrusion apparatus and increases the manufacturing cost.